1. Field of the Invention
The present invention relates to novel catalysts for the polymerization of olefins which comprise particulates of a prepolymer prepared, for example by the prepolymerization of ethylene, either alone or in admixture with an alpha-olefin having from 3 to 8 carbon atoms, in the presence of a particulate catalyst system including, on the one hand, a catalyst component containing titanium, magnesium, chlorine and, where appropriate, an electron donor or electron acceptor and, on the other hand, a cocatalyst. Such catalyst, essentially in the form of the prepolymer, is useful for the polymerization of olefins in the absence of supplementary added cocatalyst; it is characterized in that it has a particle size distribution narrower than the particle size distribution of the catalyst component, per se.
2. Description of the Prior Art
It is known to this art that during the polymerization of olefins, particularly in the instance of conducting the polymerization in the gas phase, in the presence of a solid particulate catalyst, such as those based on titanium supported on magnesium chloride, the constituent particulates are reduced in size into finely divided catalyst particles, whether by bursting or by wear/attrition, which finely divided particles promote the formation of correspondingly finely divided polymer particles.
Too, in the gas phase polymerization reactor, it is these fine particles which become the most highly charged with static electricity. These highly electrostatically charged fine particles stick to the reactor walls and promote the formation of crusts by gelling. The microfine particles also have a tendency to disengage from the fluidized bed, to promote plugging in the recycling circuits. Finally, in powder transport systems, and fine particles also become charged by static electricity and may present a risk of violent electrostatic discharge during transfers.
To overcome the above disadvantages, it has been proposed to convert the catalyst component into prepolymer form. In this event, an olefin is prepolymerized under mild conditions, in the presence of such catalyst component, to provide a low degree of polymerization progression, on the order of a few tens of grams of polyolefin formed per gram of catalyst component. This active prepolymer is then used as a catalyst system for the polymerization of olefins. Such technique permits consolidating the catalyst component, thus reducing the subsequent formation of finely divided polymer particles over the course of polymerization. However, this technique does not permit removing the fine particles initially present in the catalyst component or formed during prepolymerization, despite taking all precautions.
Ideally, a catalyst particle effects formation of a prepolymer particle which, in turn, is converted into a polymer particle. During the polymerization, the catalyst particle grows, essentially proportionally to the (productivity).sup.1/3. The particle size distribution curves of the catalyst and of the polymer are homothetic. Thus, the smaller the proportion of fine particles contained in the catalyst component, the smaller the amount of fine particles in the final polymer. These homothetic particle size distributions may be defined by a mathematical expression which makes it possible to quantify the particle size distribution or the range which is designated SPAN. The SPAN corresponds to ##EQU1## with D90, D50 and D10 being the respective diameters below which there are 90%, 50% and 10% by weight of the particles, respectively.